1. Field of the Invention
The present invention relates to a positive electrode having a polymer film and a lithium-sulfur battery employing the positive electrode, and more particularly, to a positive electrode having an ionically conductive polymer film coated thereon and a lithium-sulfur battery employing the positive electrode, which has improved cycle life and battery capacity characteristics and a reduced swelling phenomenon.
2. Description of the Related Art
The rapid development of portable electronic devices has led to an increasing demand for secondary batteries having a lighter weight, a smaller size and a higher energy density. To satisfy these demands, a need exists for development of cheaper, safer and more environmentally friendly batteries.
Among the currently developing batteries satisfying such requirements, a lithium-sulfur battery is one of the most promising batteries in view of energy density and environmental friendliness. With respect to specific energy density, the lithium-sulfur battery is the most desirable since sulfur has an energy density of 1,675 mAh/g, which is about 8 times higher than that of lithium cobalt oxide (LiCoO2) or lithium manganese oxide, which have been conventionally widely used as a positive active material of a lithium battery. Further, the sulfur-based compounds are less costly than other materials and are environmentally friendly. However, no lithium-sulfur batteries have yet been commercialized.
One of the reasons these batteries have not been commercialized is the poor sulfur utilization over repeated cycling, resulting in a low capacity. The sulfur utilization is referred to as a ratio of the amount of the sulfur involved in the electrochemical redox reaction of batteries to the amount of total sulfur used.
Further, sulfur is diffused away to electrolytes upon the redox reaction, deteriorating the cycle life characteristics. Accordingly, unless the electrolyte is suitable, the reduced product of sulfur, lithium sulfide (Li2S), is precipitated, and as a result does not participate in further electrochemical reactions. That is, lithium-sulfur batteries use elemental sulfur as a positive active material at an initial stage. As the battery is continuously discharged, 8 sulfur elements present in a ring-shaped molecular state are reduced to become linear molecules until they finally become sulfur anions (S2−), which is strongly bonded to neighboring lithium cations, forming lithium sulfide (Li2S). The formed lithium sulfide (Li2S) is precipitated on a surface of a positive electrode, which reduces an active area of a battery. Also, since the precipitated lithium sulfide (Li2S) cannot be oxidized during charging, the capacity of the battery is lowered. Therefore, it is necessary to dissociate lithium sulfide to maintain an active area of the battery.
In order to increase degree of sulfur utilization and solubility of lithium sulfide (Li2S), a variety of research into lithium salts and nonaqueous solvents has been conducted, including the followings.
U.S. Pat. No. 6,030,720 discloses a lithium-sulfur battery using a mixed electrolyte solvent including, as a main solvent, an ethoxy repeating unit compound of the general formula R1(CH2 CH2O)nR2, where n ranges from 2 to 10, R1 and R2 are different or identical alkyl or alkoxy groups (including substituted alkyl or alkoxy groups) and a cosolvent having a donor number of at least about 15. R1 and R2 may together with (CH2 CH2O)n form a closed ring. Also, the disclosed battery has a separation distance of not greater than about 400 micrometers.
In order to achieve a battery having improved cycle life and safety, U.S. Pat. No. 5,961,672 discloses an electrochemical battery cell comprising a stabilized lithium anode using a thin film of a lithium ion conducting polymer.
U.S. Pat. No. 5,523,179 discloses a lithium-sulfur battery having a positive electrode including about 20 to 80 wt % of active-sulfur, about 15 to 75 wt % of an ionically conductive material and about 5 to 40 wt % of electronically conductive material.
U.S. Pat. No. 5,814,420 discloses a lithium-sulfur battery comprising a positive electrode including an electrochemically active material containing sulfur present in forms of elemental sulfur, lithium sulfide and lithium polysulfide, and an electronically conductive material.
Many attempts have been made to overcome several drawbacks of the conventional lithium-sulfur batteries. Nevertheless, the key problem with the conventional lithium-sulfur battery, that is, low degree of sulfur utilization, is still open for resolution.
Another problem with the conventional lithium-sulfur battery is deterioration in cycle life and capacity characteristics due to use of lithium metal as a negative electrode. That is, with repeated charge and discharge cycles, dendrites grow on a surface of lithium metal to contact the positive electrode, which leads to short circuit of the battery, impairing battery performance. Deterioration in battery capacity results from erosion of lithium metal due to a reaction between the lithium surface and an electrolytic solution.
To resolve these problems, a method of forming a protective film on a surface of a lithium electrode has been proposed, as disclosed in U.S. Pat. Nos. 6,017,651, 6,025,094 and 5,961,672.
In order to ensure proper operation of the lithium protective film, it is necessary to prevent lithium and electrolyte from contacting each other while allowing lithium ions to migrate freely. The prior art, however has several problems.
That is, most of lithium electrode protective films, which are formed after assembling the battery, followed by reacting lithium with additives in an electrolytic solution, have poor density, so that a considerable amount of the electrolytic solution permeates through pores in the protective film, undesirably resulting in contact with lithium metal.
Alternative way of forming a lithium protective film involves processing the surface of a lithium electrode with nitrogen plasma to form lithium nitride (Li3N) film on the lithium electrode. However, this attempt still has several drawbacks in that the lithium nitride film has grain boundaries through which the electrolytic solution easily permeates, and has so poor resistance to water that it is liable to decompose when in contact with water. Also, since the lithium nitride film has a low potential window, i.e., 0.45 V, it is impractical to use the lithium nitride film.